In CDMA (code division multiple access) systems, the modem is the heart of the BTS (basestation transceiver subsystem). The modem performs the base band CDMA signal processing that converts data received from the BSC (basestation controller) to the CDMA air interface format on the forward link (the transmitter), and it receives the reverse link CDMA air interface signals from the mobile and converts the information into a format that the BSC understands. There is modem in every CDMA BTS regardless of manufacture.
Typically, a single BTS is designed to support multiple users. Furthermore, the BTS may need to be designed to support more than one of the various different air interface standards currently available. Current modems each process a single user for a single standard. If multiple users or multiple standards are to be supported by a BTS, multiple modems are required, one for each user of a particular standard. Thus, BTS complexity scales directly in proportion to the number of users and standards supported. Typically, BTS modems are designed on a single chip. Recently, chips have been designed which simply replicate the functionality of a single modem to create multiple-modem chips. Replicating a single modem on a chip does not alter this direct proportionality factor, it only shrinks the size of the system packaging.